System and method for wearable, ubiquitous RFID-enabled sensing

ABSTRACT

A system includes a plurality of radiofrequency identification (RFID) tags including a first RFID tag. The first RFID tag is configured to perform RFID tag operations that include acquiring one or more samples using a sensor of the first RFID tag. The RFID tag operations also include writing the one or more samples in a memory of the first RFID tag. The RFID tag operations also include transferring custody of the one or more samples to a first RFID reader on request. The system also includes a plurality of RFID readers including the first RFID reader. The first RFID reader is configured to perform RFID reader operations including reading an identifier from each of the plurality of RFID tags in view of the first RFID reader during an inventory management mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Provisional Patent Application No.62/928,837, filed on Oct. 31, 2019, the entirety of which isincorporated by reference herein.

GOVERNMENT RIGHTS

The invention described herein was made by employees of the UnitedStates Government and may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND

Wireless systems and methods may be used in vehicles, buildings, towers,appliances, etc. to provide communication at a reduced overall weight(e.g., due to the lack of wires). Wireless communication also affordsthe flexibility to add wearable, on-body sensors without constraining awearer's range of motion by virtue of a cable harness acting as atether.

NASA has been interested in wirelessly monitoring the environment in theimmediate vicinity of its crew members aboard the International SpaceStation (ISS), especially for the study of potentially hazardoussituations such as the aggregation of CO₂ gas bubbles in microgravity.One related approach by NASA for wearable sensors in a wirelesscommunication system uses Bluetooth Low Energy (BLE) on-orbit. Thisapproach gathers useful data, but the reliance on commercialoff-the-shelf (COTS) active wireless protocols such as BLE limits theoperational lifetime of such system, currently rated at a few days on arechargeable battery. Accordingly, there is a long-felt need for animproved wearable/peel-and-stick system and method for wirelesscommunication.

SUMMARY

A system is disclosed. The system includes a plurality of radiofrequencyidentification (RFID) tags including a first RFID tag. The first RFIDtag is configured to perform RFID tag operations that include acquiringone or more samples using a sensor of the first RFID tag. The RFID tagoperations also include writing the one or more samples in a memory ofthe first RFID tag. The RFID tag operations also include transferringcustody of the one or more samples to a first RFID reader on request.The system also includes a plurality of RFID readers including the firstRFID reader. The first RFID reader is configured to perform RFID readeroperations including reading an identifier from each of the plurality ofRFID tags in view of the first RFID reader during an inventorymanagement mode. The RFID reader operations also include transmittingthe identifier from each of the RFID tags in view to a sensor controllerduring the inventory management mode. Transmitting the identifierincludes transmitting a first identifier for the first RFID tag to thesensor controller. The RFID reader operations also include receiving afirst custody transfer command from the sensor controller based at leastin part on the sensor controller receiving the first identifier. Basedat least partially upon receiving the first custody transfer command,the RFID reader operations also include reading the one or more samplesfrom the first RFID tag to take custody of the one or more samplesduring a custody transfer servicing mode. During the custody transferservicing mode, the RFID reader operations also include writing a secondcustody transfer command to the memory of the first RFID tag to indicatethat the first RFID reader has taken custody of the one or more samples.The sensor controller is configured to be in communication with theplurality of RFID readers. The sensor controller is configured toperform sensor controller operations including receiving the firstidentifier from the first RFID reader during the inventory managementmode. The sensor controller operations also include determining that thefirst RFID tag is in view of the first RFID reader based on receivingthe first identifier. The sensor controller operations also includetransmitting the first custody transfer command to the first RFID readerto cause the first RFID reader to request the one or more samples fromthe first RFID tag.

A method is also disclosed. The method is for the collection ofinformation from a plurality of radio-frequency identification (RFID)tags using an RFID system. The method includes scanning for theplurality of RFID tags with a plurality of RFID readers. The pluralityof RFID readers includes a first RFID reader having a first coveragearea and configured to detect one or more of the plurality of RFID tagswhen positioned in the first coverage area. The method also includesreceiving RFID tag identifiers corresponding to the plurality of RFIDtags with the first RFID reader. The RFID tag identifiers include afirst RFID tag identifier corresponding to a first RFID tag of theplurality of RFID tags. The method also includes transmitting the RFIDtag identifiers from the first RFID reader to a sensor controller. Themethod also includes identifying the first RFID tag identifier in theRFID tag identifiers using the sensor controller. The method alsoincludes determining that the first RFID tag is likely to have custodyof sensor data. The method also includes issuing a first custodytransfer command from the sensor controller to the first RFID readerbased at least in part on the determination that the first RFID tag islikely to have custody of the sensor data. The method also includesreading the sensor data from the first RFID tag using the first RFIDreader in response to the first custody transfer command. The methodalso includes writing a second custody transfer command from the firstRFID reader to the first RFID tag.

In another embodiment, the system includes a radiofrequencyidentification (RFID) tag configured to perform RFID tag operations. TheRFID tag operations include transferring custody of data in a packetbuffer of a memory of the RFID tag to an RFID reader, thereby freeingthe packet buffer to receive new data. The RFID tag operations alsoinclude acquiring one or more samples using a sensor of the RFID tag.The RFID tag operations also include storing the one or more samples ina sample buffer of the memory of the RFID tag. The RFID tag operationsalso include removing a first entry in the sample buffer when the firstentry contains greater than or equal to a threshold number of the one ormore samples. The RFID tag operations also include encapsulating thefirst entry as a packet. The packet includes the one or more samples andpacket metadata. The RFID tag operations also include placing the packetin the packet buffer in response to a custody transfer request from theRFID reader. The RFID tag operations also include adding an empty entryto the packet buffer when the sample buffer contains less than thethreshold number of the one or more samples. The RFID tag operationsalso include allowing acquisition of new samples into a second entry inthe sample buffer concurrently with processing a new custody transferrequest from the RFID reader.

This summary is provided to introduce a selection of concepts that arefurther described below in the detailed description. This summary is notintended to identify key or essential features of the claimed subjectmatter, nor is it intended to be used as an aid in limiting the scope ofthe claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures form part of the present specification and areincluded to further demonstrate certain aspects of the presentlydescribed subject matter and should not be used to limit it. The presentsubject matter may be better understood by reference to one or more ofthese drawings in combination with the description of embodimentspresented herein. Consequently, a more complete understanding of thepresent embodiments and further features and advantages thereof may beacquired by referring to the following description taken in conjunctionwith the accompanying drawings, in which like reference numerals mayidentify like elements, wherein:

FIG. 1 illustrates a schematic view of a radio frequency identification(RFID)-enabled information collection system including an RFIDreader/interrogator, an RFID tag, and an RFID sensor controller,according to an embodiment.

FIG. 2 illustrates a schematic view of the RFID reader of FIG. 1,according to an embodiment.

FIG. 3 illustrates a schematic view of the RFID tag of FIG. 1, accordingto an embodiment.

FIG. 4 illustrates a schematic view of another RFID tag (similar to theRFID tag in FIG. 3) including a distinct integrated circuit connectedvia a serial interface to a low-power microcontroller unit (MCU),according to an embodiment.

FIG. 5 illustrates a schematic view of the RFID sensor controller ofFIG. 1, according to an embodiment.

FIGS. 6A and 6B illustrate a flowchart of method for a data acquisitionprotocol for the RFID tag, according to an embodiment.

FIG. 7 illustrates a flowchart of a method for using the sensorcontroller, according to an embodiment. More particularly, the methodmay use an RFID sensor controller to subscribe to the interrogationfeeds from a networked set of RFID interrogators, determine if an RFIDtag of interest is in the interrogation feed, determine if the RFID tagof interest is likely to have custody of sensor data, to determine anoptimal access strategy (including selecting an RFID interrogator), andsubsequently send a message to an RFID interrogator commanding it totake custody of the RFID tag's stored sensor data.

FIGS. 8A and 8B illustrate a flowchart of a method for using the RFIDreader, according to an embodiment. More particularly, the method mayuse the RFID reader to alternate between (1) interrogating and streamingtag interrogation results, and (2) receiving and processing commands toorder an RFID sensor tag to transfer custody of its stored sensor datato the RFID interrogator, according to an embodiment.

FIG. 9 illustrates a flowchart of a store-and-forward data transmissionprotocol for the RFID tag, according to an embodiment.

DETAILED DESCRIPTION

Reference may now be made in detail to specific embodiments illustratedin the accompanying drawings and figures. In the following detaileddescription, numerous specific details are set forth in order to providea thorough understanding of the embodiments described herein. However,it may be apparent to one of ordinary skill in the art that otherembodiments may be practiced without these specific details. In otherinstances, well-known methods, procedures, components, circuits, andnetworks have not been described in detail so as not to unnecessarilyobscure aspects of the embodiments.

It may also be understood that, although the terms first, second, etc.may be used herein to describe various elements, these elements shouldnot be limited by these terms. These terms are only used to distinguishone element from another. For example, a first object could be termed asecond object, and, similarly, a second object could be termed a firstobject, without departing from the scope of the present disclosure.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used in thedescription and the appended claims, the singular forms “a,” “an” and“the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. It may also be understood that theterm “and/or” as used herein refers to and encompasses any and possiblecombinations of one or more of the associated listed items. It may befurther understood that the terms “includes,” “including,” “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, operations, elements, components,and/or groups thereof. Further, as used herein, the term “if” may beconstrued to mean “when” or “upon” or “in response to determining” or“in response to detecting,” depending on the context.

As used herein, the terms “inner” and “outer”; “up” and “down”; “upper”and “lower”; “upward” and “downward”; “above” and “below”; “inward” and“outward”; and other like terms as used herein refer to relativepositions to one another and are not intended to denote a particulardirection or spatial orientation. The terms “couple,” “coupled,”“connect,” “connection,” “connected,” “in connection with,” and“connecting” refer to “in direct connection with” or “in connection withvia one or more intermediate elements or members.”

The embodiments described herein include systems and methods that useradio frequency identification (RFID) technology to transport data to anRFID inventory system. In one embodiment, the RFID inventory system maybe aboard the ISS and may exploit the inherently passive nature of RFIDsensor tags to provide wearable or structure-mountable, wireless sensorsthat are capable of operating for one or more years on small coinbattery cells. The embodiments described herein include astore-and-forward overlay on RFID reader and RFID sensor tag protocolsthat allow an RFID sensor tag to transmit signals through a system ofRFID interrogators (also referred to as RFID readers), exploitingcontact opportunities as they arise, and quietly transferring sensorreadings at nearly no power cost to the wireless RFID sensor itself.This approach essentially zeros-out the largest element of the powerbudget of a typical Internet of Things (IoT) sensor. The embodimentsdescribed herein also include wearable RFID sensors using commerciallyavailable components that have been modified to become part of a systemand method for wearable, ubiquitous RFID-enabled sensing that isimminently manufacturable.

In one embodiment, the RFID system may be configured to automatecollection of data and may include a plurality of RFID interrogators anda plurality of RFID tags. Each RFID tag may have at least one processorand memory for (1) storing a set of instructions and data and (2)implementing over-the-air RFID services (e.g., Electronic Product Code(EPC) memory and User Memory (UM) in EPC Global Class 1 Generation 2).When the set of instructions is executed by the at least one processor,it causes the RFID tag to perform operations. The operations include:(a) on command, sampling by collecting data into a “sample buffer”section of the memory; (b) loading a first packet of data into a “packetbuffer” (primary portion; accessible over-the-air, e.g., UM) of the RFIDtag memory for collection by one of the plurality of RFID interrogatorsonce a packet worth of data has been collected; (c) continue collectingadditional samples of data into the sample buffer (e.g., secondaryportion; not accessible over-the-air) of the RFID tag memory to be usedto form “subsequent” packets, allowing continuous data gathering, evenwhile the primary portion of the RFID tag memory is filled with apacket; (d) maintaining custody of the subsequent packets until the RFIDtag has been explicitly informed of a custody transfer of the firstpacket of data by one of the plurality of RFID interrogators; (e)building each subsequent packet using samples from the sample bufferaccording to a prioritization scheme, and loading the packet into thepacket buffer once custody of the first packet of data stored in theprimary buffer section has been transferred to the RFID interrogator;and (f) maintaining a time-base estimate for time-tagging the packets ofdata collected as samples and processing time-base updates from the RFIDinterrogator.

In another embodiment, the RFID system may be configured to automatecollection of data. The system may include at least one RFID sensor tagand a plurality of RFID readers. Each RFID reader may have at least oneprocessor and memory system for storing a set of instructions. The setof instructions on the RFID reader, when executed by the at least oneprocessor, causes the RFID reader to realize that an RFID sensor tag isin range of signal reception (e.g., either on its own initiative orthrough notification from an RFID reader coordinator). The set ofinstructions also causes the RFID reader to determine whetherunrecovered sensor data is present on the RFID sensor tag and to take“custody” of a “packet” of sensor data on the RFID tag identified asbeing within range of signal reception. This step of taking custody mayinclude, as an example, informing the RFID sensor tag that custody hasbeen transferred and instructing the same RFID sensor tag to queue up asecond packet of data. The set of instructions may optionally update thetime-base of the RFID sensor tag or execute other procedures.

In another embodiment, a method is disclosed for using a system of RFIDinterrogators and/or readers to support the collection of sensor datafrom ultra-low power, wearable or surface-mountable, moveable RFIDsensor tags. The method uses a store-and-forward approach to manage thecollection of data from the RFID sensor tags, even when they are not inrange of an individual RFID interrogator, and as they move from thecoverage area of one RFID interrogator to the next in a complexstructure such as the ISS, a hospital, or an industrial facility. Thismethod allows use of RFID for transporting sensor data in a complexenvironment where instantaneous access to an RFID interrogator cannot beguaranteed, extending RFID data collection to mobile, wearable sensing.By using RFID systems and methods for data transport and off-loading theconsumption of power for communication from the wireless sensor tag, themethod enables wearable RFID-enabled sensing with years-long lifetimeson small, consumable coin cell batteries.

The system and method disclosed herein manage automatic downloading ofRFID sensor data logs within a multiple-RFID interrogator ecosystem. Thesystem and method do not require explicit wearer-user intervention. Thesystem and method also allow any one of a multiplicity of networked RFIDinterrogators to communicate with an RFID tag and transfer logged datastored on such tag. The system and method also support a high amount ofmobility of the RFID tag, both when located within the broad coveragearea of the networked RFID interrogators and when located outside theRFID interrogator coverage area. The system and method also areinterleaved with inventory management interrogations, such that they donot significantly impact the primary interrogation task. The system andmethod also use all standard EPC Global Class 1 Generation 2 (C1G2)commands when implemented using the C1G2 protocol, which means they doesnot require any proprietary command-set extensions.

In another embodiment, the system and method may treat the user memory(UM) of the RFID tag as a delay-tolerant radio link. The system andmethod also include a set of instructions on the RFID interrogator sideto:

-   -   a. realize that an RFID tag is in view (either on its own or        through notification from an interrogator coordinator);    -   b. determine that unrecovered sensor data is present on the RFID        tag;    -   c. take “custody” of a “packet” of sensor data on that tag        (e.g., inform the tag that custody has been transferred and        instruct it to queue up the next packet of data); and    -   d. optionally update the time-base of the tag or execute any        other required procedures (e.g., stop sampling, re-configure        sampling period, etc.).

The system and method also include a set of instructions on the RFID tagside to:

-   -   a. on command, begin sampling data into a buffer organized into        “packets” of data;    -   b. load a packet into primary (“packet buffer”) tag memory for        collection by an interrogator if a packet worth of data has been        collected;    -   c. continue sampling into a secondary buffer (“sample buffer”)        of subsequent packets, allowing continuous data gathering even        while the primary tag memory is filled;    -   d. maintain custody of the data in the packet buffer until it        has been explicitly informed of a custody transfer by an        interrogator;    -   e. load the “next” packet's worth of samples from the sample        buffer into the packet buffer in the tag memory (according to        some prioritization scheme, e.g., packet age) when custody of        data in the packet buffer has been transferred to the        interrogator; and    -   f. maintain a time-base estimate for time-tagging samples and        provide a method for allowing the interrogator to update the        time base estimate.

The system and method also include a set of instructions for individualRFID interrogators in a multi-interrogator system to populate a commondatabase of sensor readings without needing to know details of the lastRFID interrogator/tag interaction. This may include a sensor coordinatorobserving inventory management tag reads from the population ofinterrogators, and scheduling data transfers for individual tags withindividual interrogators. This may optionally useinterrogator/antenna/frequency information from inventory managementinterrogations to seed the “custody transfer” process.

The systems and methods disclosed herein include an RFID reader (alsoreferred to herein as an RFID interrogator) and an RFID tag that provideRFID-enabled information collection that can be used for monitoring andgathering data wirelessly from a host structure. In an embodiment, theRFID tag may be a wearable tag that is positioned on or near a humanbody (e.g., coupled to clothing). Examples of a host structure may be orinclude the structural members, interior walls, or equipment of a crewedor robotic vehicle (e.g., a spacecraft) or similar elements of a spacehabitat. Such systems and methods may operate at low power for longdurations by implementing a first protocol for offline data gatheringand a second protocol for low-power data transfer.

Transmission of data is managed by a network of RFID interrogatorsrunning a store-and-forward software overlay that is compatible withsimilar software running on the RFID sensor tag as well as a centralizedcontroller commanding interrogators and tags to interact at moments ofopportunity. Below, the tag, the interrogator, and the sensor controllercomponents of this system are described.

FIG. 1 illustrates a schematic view of an RFID-enabled informationcollection system 100, according to an embodiment. The system 100includes a sensor controller 500. The system 100 also includes one ormore RFID readers (also referred to herein as an RFID interrogators)200. The RFID readers 200 may be in wired or wireless communication withthe sensor controller 500. The system 100 also includes one or more RFIDtags 300. The RFID tags 300 may be in wireless communication with theRFID readers 200. As described below, the system 100 may be configuredto collect/acquire, store, receive, and/or transmit time-series data(“forward”) from each of a plurality of sensor devices.

FIG. 2 illustrates a schematic view of the RFID reader 200, according toan embodiment. The RFID reader 200 may include a central processing unit(CPU) or microprocessor 210, a communication bus 214, at least one RFIDinterrogator module 220, at least one RF antenna 230, a digital datacommunications port or backhaul network 240, and a power source 250.

The processor 210 may include a set of instructions for implementing andperforming the steps of functionality outlined below. The processor 210may be embodied as an application-specific integrated circuit (ASIC)chip, a field programmable gate array (FPGA), a microcontroller unit(MCU), a single board computer, or such other computing mechanismcapable of storing in a memory integrated thereto a set of instructionswritten in human-readable format (e.g., source code, VHDL) andconverting such instructions into a set of machine-readable digitalinstructions (e.g., binary code) for operating the RFID reader 200.

The at least one RFID interrogator module 220 may include either adistinct integrated circuit connected via the bus 214 (e.g., serialinterface) or a circuit module internal to the processor 210. Each RFantenna 230 may interface to/with the RFID interrogator module 220 andmay be configured to send command sequences to the RFID tag 300. The RFantenna 230 may also be configured to receive transmitted informationcollected and transmitted by the RFID tag 300.

The digital data communications port or backhaul network 240 may beoperatively connected to the processor 210 and the RFID interrogatormodule 220 to “offload” collected sensor data that is either wired(e.g., via Ethernet) or wireless (e.g., via Wi-Fi).

FIG. 3 illustrates a schematic view of the RFID tag 300, according to anembodiment. The RFID tag 300 may include a tag central processor unit(CPU) or microcontroller unit (MCU) 310 having a memory 312. The RFIDtag 300 may also include at least one sensor element 320 for sensing andcollecting a data set of information (e.g., a physical measurement). Thesensor element 320 may be or include, for example, a thermocoupleelement, a strain gauge, a carbon dioxide sensor, a light sensor, atemperature sensor, a humidity sensor, an air quality sensor, alow-bandwidth accelerometer, or the like. The RFID tag 300 may alsoinclude a communications bus 330, a power source 340, and at least onetransceiving antenna 350.

The MCU 310 includes a set of instructions stored on its memory 312 tocause the RFID tag 300 to perform a set of steps as may be described infurther detail below with reference to FIGS. 6A, 6B, and 9. Each sensorelement 320 may be configured to sense a parameter of interest to enablethe collection of a set of information. For example, the parameter ofinterest may be or include temperature when the sensor element is athermocouple, the presence of a chemical when the sensor is chemicalsensor, strain when the sensor is a strain gauge. The MCU 310 mayfurther include at least one RFID tag module 315 for implementing awakeup channel. The at least one RFID tag module 315 may be configuredto generate a wakeup pulse signal when a particular data sequence istransmitted by the RFID reader 200 to the RFID tag module 315.

The transceiving antenna 350 may be connected to the RFID tag module 315for receiving data sequences and for transmitting a set of informationcollected from the at least one sensor element 320. The power source 340may maintain the tag state when not under interrogation by the RFIDreader 200. In one embodiment, the power source 340 may be or include aconsumable battery. In another embodiment, the power source 340 may beor include a power harvester coupled with a power storage device. In yetanother embodiment, the power source 340 may be or include a combinationof both a consumable power supply and a power harvester.

Accordingly, RFID tag operation (e.g., hibernating, wakeup, and/orsampling) may be powered entirely by using power harvested from the datasequence transmitted by the RFID reader 200. In such an embodiment, thepower source 340 may be configured to harvest power from the transmittedsignal of the RFID reader 200 or another source of ambient energy (e.g.,thermal gradients). In another embodiment, a portion of the operatingpower may be generated by using power harvested from the data sequenceof the RFID reader 200, with the remaining power being provided byanother power supply integrated with a power harvester contained withinpower source 340, such as a battery. In yet another embodiment, thepower source 340 may include a battery without a power harvester.

The RFID tag module 315 may be an integrated circuit module internal tothe MCU 310. In another embodiment, the RFID tag module 315 may be adistinct integrated circuit connected via a circuit bus (e.g., serialinterface) to the MCU 310, similar to the configuration shown in FIGS.6A and 6B, which are described below.

FIG. 4 illustrates a schematic view of another RFID tag 400 (similar tothe RFID tag 300 in FIG. 3) including a distinct integrated circuitconnected via a serial interface to a low-power MCU, according to anembodiment. The RFID tag 400 may include at least one RFID tag module415 for implementing a wakeup channel. The RFID tag 400 may also includean MCU 410 configured to periodically wake itself from a low power state(e.g., either on a schedule or driven by an external interrupt). The MCU410 may also be configured to monitor the RFID tag module 415 for thepresence of a particular data sequence. The RFID tag module 415 may be adistinct integrated circuit connected via a circuit bus 430 (e.g.,serial interface).

The RFID tag 400 may use a new class of serial-addressable EPC GlobalC1G2 RFID integrated circuits (ICs) for the RFID tag module 415. Such anIC, in addition to being a C1G2-compliant RFID device, adds a serialinterface (e.g., SPI, I2C, UART) through which the C1G2 memory banks canbe read or written by an attached device, such as a smallmicrocontroller. The RFID reader 200 may provide the power to read/writea tag memory 414 using the OTA interface. The attached processor 410provides the set of instructions stored on a processor memory 412 toread/write tag the memory 414 using the serial interface and configureinterrupt-based wakeup should the service be provided by the RFID IC.When neither interface is engaged, the RFID IC may be completely powereddown.

Though the memory architecture of the RFID tag 300, 400 may vary (e.g.,EEPROM, FRAM, etc.), all serial-addressable tags may share one commonfeature: reading from and/or writing to the tag memory 414 consumesrelatively little power compared to the power draw of activetransmitter/receiver protocols such as BLE, ZigBee, and Wi-Fi. Thus, theRFID reader 200 can write data using the OTA interface to the RFID tagmemory 414 using only power from the RFID reader 200. Then, with only asmall cost to the integrated tag/processor device's power supply 440,the processor 410 can read the data out over the serial interface.

The RFID tag 400 may also include at least one RF transceiving antenna450 (two are shown) operatively connected to the RFID tag module 415 forreceiving data sequences and/or for transmitting a set of informationcollected from the at least one sensor element 420. As described earlierwith reference to the RFID tag 300 of FIG. 3, the RFID tag 400 may alsoinclude a power source or supply 440 to maintain the tag state when notunder interrogation by the RFID reader 200. In one embodiment, the powersource 440 may be a consumable battery. In another embodiment, the powersource 440 may be a power harvester coupled with a power storage device.In yet another embodiment, the power source 440 may be a combination ofa both consumable power supply (e.g., a button battery) and a powerharvester.

The embodiments disclosed herein may use a serial-addressable RFIDinterface in an RFID tag 300, 400 so that the RFID reader 200 can “wakeup” a “sleeping” wireless sensor 320, 420 that has been placed intolong-term, low-power hibernation when instructed to do so by the sensorcontroller 500. The wireless sensor 320, 420 may be part of the RFID tag300, 400.

As mentioned above with reference to FIG. 3, the (e.g., wireless) RFIDtag 300 may include a low-power microcontroller unit (MCU) 310 and aserial-addressable RFID integrated circuit (IC) (e.g., a telemetryinterface) 315 for transmitting sensor data and for implementing awakeup channel (i.e., a wakeup interface). In the hibernatingconfiguration, the MCU 310 may be the only element in the RFID tag 300that draws power, and the MCU 310 may be operating in the lowest powermode possible. In at least one embodiment, in the lowest possible powermode, either nothing happens, or a watchdog timer is monitored to ensurethat the processor can be reset in the event of a software problem. Whenit is time to bring the RFID tag 300 into an active (e.g., higher-power)state, the RFID reader 200 transmits (i.e., writes) a configurationcommand into one of the memory banks of the RFID tag 300 (e.g., the usermemory (UM) bank optionally included in the EPC Global Class 1Generation 2 (C1G2) protocol). Upon detection of this event, the MCU 310reads out the configuration command from the RFID reader 200 and engagesthe telemetry interface 315 as appropriate to the command. By using apassive channel afforded by the wakeup interface, the sensor processor310 may avoid the significant power draw of periodically turning on itswakeup channel, connecting to the RFID reader 200 (i.e., the masterdevice), and checking for pending wakeup messages.

A passive wakeup channel can also have one additional advantage: somemodels of serial-addressable RFID IC can be configured to generate a“wakeup pulse” on one of the serial input/output lines when a particulardata sequence is written over-the-air (OTA) to an RFID IC of the RFIDtag 300. In one embodiment, this pulse may be generated by using powerharvested from the RFID reader 200, and the MCU 310 can be configured togenerate and service a hardware interrupt on detection of that pulse.This configuration provides an extremely low-latency wakeup capability,because detection of the wakeup command may be handled entirely via thehardware. Conventional models lack this feature, which means the MCU maybe configured to periodically wake up and read the contents of a fixedblock of tag memory for new configuration commands. Although this typeof embodiment cycles the MCU 310 through its active state more often,the power required to do so is much less than if an active radio channelwere powered on as well. Also, the time taken to read the configurationregister may be much shorter than the connection time of a protocol suchas Bluetooth® Low Energy (BLE), so the average time spent in the activeperiod checking for new configuration commands is much shorter. Thewakeup period can be adjusted to provide the desired latency in decodingand acting upon configuration commands.

The MCU 310, 410 has a memory 312, 412 that includes a set ofinstructions for receiving a time-base synchronization signal from theRFID reader 200 through the RFID module 315, 415 and updating/refiningthe time base maintained by the MCU 310, 410 using such synchronizationdata over the course of operation. The steps for maintainingsynchronization includes a step for efficiently encoding timestamps of asequence of data samples. The set of instructions also includes a methodfor maintaining concurrent acquisition of a new sequence of data sampleson the MCU 310, 410 while downloading (i.e., communicating) a previoussequence of data samples from the memory bank of the RFID module tagmemory 315, 415 to the RFID reader 200.

During data acquisition by the RFID tag 300, 400, the tag softwareincludes a set of instructions that cause the RFID tag 300, 400 toautomatically enter a low-power state when the tag is (1) not activelygathering data samples from sensor element 320, 420 and/or (2) notactively writing data samples to or reading data from the memory bank ofRFID module 315, 415.

Further, during data acquisition by the RFID tag 300, 400, the tagsoftware includes a set of instructions that cause the RFID tag 300, 400to automatically return to a high-power state per the data samplingschedule to (1) gather data samples from the at least one sensor element320, 420 and/or (2) write data samples to the RFID tag memory 312, 412.

FIG. 5 illustrates a schematic view of the sensor controller 500,according to an embodiment. The sensor controller 500 may include acentral processing unit (CPU) or microprocessor 510, a communication bus514, a digital data communications port or backhaul network 540, and apower source 550.

FIGS. 6A and 6B illustrate a flowchart of a method 600 for a dataacquisition protocol for the RFID tag 300, 400, according to anembodiment. The steps in the method 600 may be performed by the RFIDtag(s) 300, 400. The method 600 may include placing/switching the RFIDtag 300, 400 into a low-power (e.g., inactive) state, as at 602. In thelow-power state, the RFID tag 300, 400 may be sleeping and/orhibernating. Once RFID the tag 300, 400 is in the low-power state,method 600 may also include determining whether a wake-up signal hasbeen received by the RFID tag 300, 400 from the RFID reader 200, as at604. If no wake-up signal has been received, the RFID tag 300, 400 mayrevert back and/or remain in the low-power state. If the wake-up signalhas been received, the tag 300, 400 may switch into an active statewhere it is configured to extract, decode, and/or interpret a commandembedded in the wake-up signal, as at 606.

The method 600 may also include determining whether the command encodesan aperiodic action or a periodic action, as at 608. An aperiodic actionmay include, for example, generating a calibration value set orgenerating a health and status report. A periodic action may include,for example, periodic sensor sampling using the sensor element 320, 420.If the command encodes an aperiodic action, the RFID tag 300, 400 mayperform that action and then return to sleep, as at 610. However, if thecommand encodes a periodic action, a sampling clock of the processor310, 410 in the RFID tag 300, 400 may synchronize with a clock in theRFID reader 200, as at 612.

The RFID tag 300, 400 may then begin acquiring samples using the sensor320, 420. This may include starting a sample timer in the RFID tag 300,400 using the synchronized sampling clock to govern when samples are tobe taken, as at 614. The method 600 may also include adding an emptyentry to a packet buffer in the primary area of the RFID memory 312,414, as at 616. As used herein, an “empty entry” refers to an entryindicating that no samples are present in a packet. As used herein, a“packet buffer” refers to data that is readable over the air via RFIDprotocols containing samples and metadata encapsulated as a packet. Inother words, a packet includes one or more samples with optional headerand footer data. As used herein, a “sample buffer” refers to data thathas been gathered in internal memory and is not readable over the airvia RFID protocols. The packet buffer may also be referred to as theprimary area of the memory 312, 414. The sample buffer may also bereferred to as the secondary area of the memory 312, 412.

The method 600 may also include setting a new entry as a current entryin the sample buffer in the secondary area of the RFID memory 312, 412,as at 618. As used herein, a “new entry” refers to an entry formatted toindicate that no samples have yet been populated into it.

The method 600 may also include writing a sample (or next sample) fromthe sensor 320, 420 into the current entry in the sample buffer when thesample timer expires, as at 620. In an embodiment, a sample may be orinclude a measurement by the sensor 320, 420. Simultaneously, thecurrent time base estimate of the sensor will be updated to include thesample timer interval.

The method 600 may also include determining whether a predeterminednumber of samples has been acquired in the current entry in the samplebuffer in the secondary area of the RFID memory 312, 412, as at 622. Ifthe predetermined number of samples has not been acquired, the method600 may loop back around to the previous step. If the predeterminednumber of samples has been acquired, then the method 600 may includedetermining whether the packet buffer in the primary area of the memory312, 414 of the RFID tag 300, 400 has a packet (e.g., including thesamples) in custody, as at 624. If the packet buffer of the memory 312,414 has the packet (e.g., including its samples and metadata) incustody, then the method 600 may loop back around to step 618. If thepacket buffer of the memory 312, 414 does not have the packet (e.g.,including its samples) in custody, then the method 600 may includeremoving a buffer entry from the sample buffer memory 312, 412,encapsulating the buffer entry as a packet, and writing the packet tothe packet buffer in the primary area of the memory 312, 414 of the RFIDtag 300, 400, as at 626.

The method 600 may also include collecting a next sample (e.g., a CO₂sample) using the sensor 320, 420 (as at 618) after having first drawn anew sample buffer entry, in response to the sample timer expiring. Inanother embodiment, the method 600 may be used to gather aperiodic databy generating a sample only when an event of interest has happened sincethe last timer expiration.

The tag software also includes a set of instructions to cause the RFIDtag 300, 400 to (1) cease acquiring data and enter an idle state and/or(2) execute any other configuration/sampling access strategy directed bythe interrogator software.

Protocol for Offline Data Gathering

Embodiments disclosed herein provide a long-lived wireless RFID-enabledsensor system that on command can begin acquiring time-tagged sensorsamples that can be made available for transfer at an arbitrarily latertime to any of a network of RFID interrogators. The sensor system may,optionally, after hibernating at low power before waking up, be orderedby an RFID interrogator in a network of such interrogators to beginacquiring time-stamped data for eventual transmission. Managing accessto each RFID tag 300, 400 may include placing a time-base estimate oneach RFID tag 300, 400 for synchronization at the outset of samplingwith the interrogator software and for updating the time-base estimateover the course of its operation. This functionality may be implementedin the sensor controller 500, encoded as a set of instructions that runin the sensor controller processor 510, prior to the iteration of thesensor controller software encoded in the flow chart of FIG. 7. Thistime-base estimate may be coincident with the command issued by thesensor controller 500 to the tag 300/400 to wake up from a low-powerhibernation state and begin sampling.

The process of receiving this command and then acting upon it isdescribed in the tag data acquisition software flow chart of method 600as depicted in FIGS. 6A and 6B. Prior to reception of the command, theRFID sensor tag 300, 400 may be in the low-power hibernation state. Fromthis state, it may wake up to receive “aperiodic” house-keeping oron-demand sampling commands, after which it may return to thehibernation state, or it may wake up to receive the “periodic sampling”command, which instructs it to begin the act of sampling a time seriesof sensor data. Once this command has been received, the RFID sensor tag300, 400 can then leave the coverage area of the network of RFID readers200 for a connection-deprived environment and still record time-stampedsensor data for later delivery once contact with the RFID interrogatorsystem has been restored. A time-base estimate may be encoded into thecommand message, and this time base, updated as samples are acquired ata fixed rate, may be used to time-tag packets of measurement data. Inone embodiment, the sample rate may be fixed. In another embodiment, itmay also be encoded in the command message along with the time-baseestimate.

Per the method 600 of FIGS. 6A and 6B, once this command has beenreceived, the RFID tag 300, 400 may synchronize its sampling clock andthen start a sample timer, upon whose expiration a set of samples may beacquired. The tag 300, 400 may maintain one “sample buffer” (or more) ofsamples to be eventually transmitted over the air as “packets” to thesystem of RFID readers 200. This sample buffer may be maintained in thememory 312, 412 internal to the sensor tag MCU 310, 410.

At the outset of sampling, or when a prior sample buffer entry has beenfilled with a set number of samples (e.g., the number of samples capableof being held in a “packet” in the packet buffer), a new buffer entrymay be added to the sample buffer(s). While a buffer entry is not yetfull, samples acquired on expiration of the sample timer may be insertedinto the buffer entry along with a time-stamp encoding their time ofacquisition.

Samples may be inserted into one or more sample buffers according tosome metric of importance. These buffers may be capable of interactingwith a separate data transmission protocol (to be discussed below aspart of the method 900) to remove sample buffer entries from the samplebuffer for transmission once the RFID tag 300, 400 is again in contactwith an element of an RFID interrogator network. Multiple samplebuffers, if implemented, may be assigned varying priorities. Both theimportance of each sample buffer and the importance of sample bufferentries within the sample buffers may be taken into account indetermining which sample buffer entries to send to the RFID interrogatornetwork first once contact has been restored. In one embodiment, thereis a single sample buffer, and the metric of performance is samplebuffer entry age as described by the time-stamps of samples in thesample buffer entry. In another embodiment, there are discrete samplebuffers for each sensor type, and priority is determined by giving somesensors attached to the device greater importance than others. Inanother embodiment, the priority is determined by some pre-processingapplied to the data itself (e.g., threshold exceedance).

When a complete sample buffer entry of samples is acquired into a samplebuffer, the RFID sensor tag 300, 400 may check to see if the RFID tagpacket buffer memory 312, 414 in the RFID module 315, 415, which isaccessible over-the-air to the network of RFID readers 200, is currentlyfilled with a packet. If it is not, the highest priority sample bufferentry may be removed from the internal sample buffer maintained in thememory 312, 412, encapsulated as a packet, and written to packet buffermemory 312, 414 in the RFID module 315, 415. If the RFID module packetbuffer memory 312, 414 is already filled with a packet, no action may betaken to remove the highest priority sample buffer entry from theinternal sample buffer, and data acquisition may proceed as describedabove with the sample buffer unmodified. Taken together, theseinstructions implement the “store” function of the store-and-forwardsuite of tag, interrogator, and controller software described herein.

Protocol for Detecting Sensor Tags and Initiating Data Transfer

Once the RFID tags 300, 400 have been commanded to begin gatheringtime-series of sensor samples, they may then be given opportunities tooffload those samples to interested clients of the data for analysis andstorage. Due to the dynamic nature of an environment where people,machinery, and inventory are constantly in motion, it is assumed thatcontact between the RFID tag 300, 400 and the RFID reader 200 initiatingthe command may be lost. In fact, it may be that sensor tag mobility maymake it likely that the RFID tag 300, 400 may find itself in the fieldsof view of other RFID interrogators long before it returns to the fieldof view of the RFID interrogator 200 that originated the command tobegin sampling. It may also be that, even if RF coverage between an RFIDtag 300, 400 and the initiating interrogator 200 is not lost, theinterrogator 200 may give priority to its “inventory management” rolefor an extended time and be disallowed by management software to enterthe data transfer subroutine, impacting data transfer the same as ifthere were a loss of RF coverage.

To accommodate this, we may provide the system with a way to recognizeand exploit “contact opportunities” between the sensor tag 300, 400,which has begun logging data, and the interrogator 200 that cansubsequently be directed to transfer those samples from the sensor tag300, 400. Taken together, these instructions implement the sensorcontroller's contribution to the “forward” function of thestore-and-forward suite of tag, interrogator, and controller softwaredescribed herein.

The system may detect that an RFID sensor tag 300, 400 with data ispresent in the fields of view of a subset of interrogators in a set ofnetworked RFID interrogators. An optimum interrogator 200 for initiatingthe custody transfer of data with the tag 300, 400 may then be selected.This protocol is resident on the sensor controller 500, which shares anetwork connection via a backhaul port 540 with a set of RFID readers200 that are themselves capable of receiving commands and streamingtelemetry on a common network using a backhaul port 240. The network ofRFID interrogators 200 may operate in the standard mode of scanning forand returning the identifiers (IDs) of tags including RFID sensor tags300, 400 they see in their field of view (e.g., the “electronic productcodes” or “EPCs” for the RFID protocol EPC Global Class 1 Generation 2).Each return contains a tag ID (e.g., an EPC), and it can optionallycontain descriptive information about the read such as received signalstrength indicator (RSSI), the RF channel on which the read occurred,the interrogator antenna on which the tag 300, 400 was read, the numberof times the tag 300, 400 was read in a unit of time, etc. These returnsare placed on the network for a centralized entity, such as an inventorymanagement system, to receive, record, and analyze.

The sensor controller 500 is responsible for observing the tag readsrecorded by the set of networked RFID interrogators and filtering forthe subset of sensor tags 300, 400. When the sensor controller 500 seesa sensor tag 300, 400 in the feed of interrogation results from thenetwork of RFID interrogators 200, it then makes a determination as towhether the RFID tag 300, 400 is likely to have logged data that needsto be offloaded. The information to make this decision may be encoded inthe tag interrogation return itself (e.g., reserved bits in the tag EPCor elsewhere in user memory to encoded presence of data). In anotherembodiment, the information to make this decision may be maintained bythe sensor controller 500 in a database (e.g., recording sample rate andlast time a sample was recovered, from which an estimate of remainingun-recovered data can be determined). Once the sensor controller 500 hasdetermined that an interrogated sensor tag 300, 400 is suitable for datatransfer, it then determines which of the RFID readers 200 that saw thetag 300, 400 should be tasked with the data recovery. This determinationmay be based on a score assigned to each interrogator's read of the tag300, 400, using such information as received signal strength indicator(RSSI), read count per unit time, load balancing between inventorymanagement and sensor data transfer at each RFID reader 200, or othersuch figures of merit. As used herein, “load balancing” refers toensuring that an individual RFID reader 200 spends a predetermined timein inventory management mode. Thus, even when a first RFID reader 200has an optimal view of an RFID tag 300, 400, a second RFID reader may beselected for the data transfer when the first RFID reader 200 hasexceeded allowable time spent outside of the inventory management modeover a given time period. As used herein, the inventory management moderefers to the mode in which the RFID reader 200 requests and returnsonly the identifiers of the RFID tags 300, 400 in its field of view, asopposed to requesting and returning any sensor data associated withthose identifiers.

FIG. 7 illustrates a flowchart of a method 700 for using the sensorcontroller 500, according to an embodiment. The steps in the method 700may be performed by the sensor controller 500. More particularly, thesensor controller 500 may include a set of instructions (“sensorcontroller software”) stored on the memory of the processor 510 in FIG.5 that may be configured such that, when executed, the sensor controller500 is caused to perform the operations described below.

The method 700 may include the sensor controller 500 receiving a list ofsensor-bearing RFID tags 300, 400 for which it is responsible, as at702. The method 700 may also include receiving (e.g., subscribing over anetwork connection to) data from the RFID readers 200 related to RFIDtag interrogations performed by the RFID readers 200, as at 704. Thesensor controller 500 may subscribe to the results while the RFIDreaders 200 are operating in an interrogate-only “inventory management”mode. In another embodiment, the method 700 may also include providingthe sensor controller 500 with some other technique for recognizingsensor tags 300, 400 in the inventory management data stream such ascommon EPC code prefixes among sensors of a similar type. This allowsthe sensor controller 500 to subsequently determine whether an RFID tag300, 400 of interest is in the field of view of one or more RFID readers200, determine if that RFID tag 300, 400 is likely to containtime-series sensor data that it has yet to offload, and make adetermination as to which of the RFID readers 200 that observed the tag300, 400 has an optimal view, according to some score. As used herein, afirst RFID reader 200 has an optimal view of the tag 300, 400 (whencompared to other RFID readers) when one or more metrics (e.g., signalstrength) describing the tag read are numerically superior for the firstRFID reader 200 (when compared to other RFID readers).

The method 700 may also include determining whether the data includesinterrogations of a predetermined number of the RFID tags 300, 400, asat 706. If the data does not include interrogations from thepredetermined number of RFID tags 300, 400, then the method 700 may loopback around to the previous step. If the data includes interrogations ofthe predetermined number of RFID tags 300, 400, then the method 700 mayinclude identifying unique identifiers of the RFID tags 300, 400 in thedata, as at 708. Said another way, this may include monitoring theresults (e.g., of inventory management RFID interrogations streamed overthe network) from individual RFID readers 200 for unique identifiers ofthe RFID tags 300, 400 to which the sensor controller 500 is subscribed.The tag streams of the RFID tags 300, 400 may be monitored for (1)presence of a subscribed tag in the field(s) of view of one or more RFIDreaders 200 and (2) evidence that the subscribed tag of interest isbearing sensor data, either as presented by the tag itself or byinformation in a database indexed by the tag's unique identifier.

The method 700 may also include determining whether a predetermined(e.g., first) RFID tag 300, 400 is detected/included in the RFID tags300, 400 based on the unique identifiers, as at 710. Said another way,this may include determining that the first RFID tag is detected basedon the results (e.g., of inventory management RFID interrogationsstreamed over the network). In one embodiment, this step may beperformed for a single RFID tag 300, 400. In another embodiment, thisstep may be performed for a plurality of RFID tags 300, 400, and thefollowing steps may be iterative. In other words, the following stepsmay be performed for each RFID tag 300, 400.

If the RFID tag 300, 400 is not detected, then the method 700 may loopback around to step 708. If the first RFID tag 300, 400 is detected,then the method 700 may include determining whether the first RFID tag300, 400 is likely to have custody of sensor data (e.g., data measuredby the sensor 320, 420), as at 712. As used herein, “likely to havecustody” refers to evidence that the subscribed tag of interest isbearing sensor data, either as presented by the tag itself or byinformation in a database indexed by the tag's unique identifier. If thedetected RFID tag 300, 400 is not likely to have custody of the sensordata, then the method may loop back around to step 708. If the detectedRFID tag 300, 400 is likely to have custody of the sensor data, then themethod 700 may include determining that a first RFID reader 200 has thebest view of the detected RFID tag 300, 400, as at 714. As used herein,the “best view” refers to appearance of the RFID tag of interest in areader's interrogation stream where such an appearance contains anelement, such as a received signal strength indicator (RSSI), that ismeasurably superior to the corresponding element in that tag'sappearance in the interrogation stream of another reader.

The responsibility for indicating that a sensor tag 300, 400 has sensordata to transfer can fall to the sensor tag 300, 400 itself, the sensorcontroller 500, or a combination of the two. In one embodiment, thesensor controller 500 modifies data that is returned during an inventorymanagement interrogation (e.g., a bit or bits of the tag EPC itself orsome small amount of user memory backscattered during a tag operationembedded in the interrogation). In another embodiment, the sensorcontroller 500 tracks the rate at which the RFID tags 300, 400 areproducing data and the time-stamp of the last data transfer to determineif new data has been collected subsequent to the data in the lastrecovered packet. In another embodiment, both preceding techniques canbe used to increase robustness.

Once an optimal RFID reader 200 is selected (e.g., the RFID reader 200with the best view), the sensor controller 500 then uses its networkinterface to issue a custody transfer command to the first RFID reader200 that best sees the RFID tag 300, 400 to begin interacting with thefirst RFID tag 300, 400 to recover at least a portion of its sensordata, as at 716. In an example, the sensor data may be loggedtime-series sensor data.

The method 700 may also include receiving the portion of the sensor dataencoded in the custody transfer from the first RFID reader 200, as at717. The method 700 may also include transmitting the portion of thesensor data from the sensor controller 500 to a database or a display,as at 718.

The steps of method 700 may be performed via human-machine interaction(e.g., via a GUI or other input control) or alternatively may beperformed automatically. In at least one embodiment, some of the stepsof the method 700 may be performed via human-machine interaction whileother steps of the method 700 may be performed automatically.

Protocol for Completing Data Transfer

Once the sensor controller 500 has identified a data-bearing sensor tag300, 400 and issued a custody transfer command as in the method 700,this command may then be intercepted by the target RFID reader 200 andacted upon using the target RFID sensor tag 300, 400. FIGS. 8A and 8Billustrate a flowchart of a method 800 describing the role of the RFIDreader 200, according to an embodiment. The steps in the method 800 maybe performed by the RFID reader 200.

At the outset of the method 800, the RFID reader 200 is operating ininventory management mode in which it reads/interrogates the RFID tags300, 400 for tag electronic product codes (EPCs) of the RFID sensor tags300, 400, as at 802. In one embodiment, this may include the RFID reader200 interrogating (e.g., performing step 802) for a predetermined amountof time (e.g., from about 10 seconds to about 30 seconds). The method800 may also include periodically transmitting results of the tag EPCinterrogation (e.g., the codes) from the RFID reader 200, over a sharednetwork connection, to the sensor controller 500, as at 804.Periodically, the RFID reader 200 may check to see if it has received acustody transfer command from the sensor controller 500 and may switchfrom periodic interrogating/transmitting to servicing the custodytransfer command, as at 806. This may be referred to as a custodytransfer servicing mode in which the RFID reader 200 begins readingsensor data associated from a tag associated with a tag identifier andwriting command data to the tag associated with the tag identifier toindicate that it has taken custody of the sensor data. In one example,this check happens periodically every 10-30 seconds. If the custodytransfer command is not received, the method 800 may loop back around tostep 802.

Upon reception of the custody transfer command, the RFID reader 200 mayextract one or more parameters from the custody transfer command, as at808. The parameters may be or include the tag identifier (e.g., EPC) andother data such as an interrogator antenna port identifier, an RFchannel, or an RF power level estimated by the sensor controller 500 togive the RFID reader 200 in question the best chance of successfullyinteracting with the RFID tag 300, 400.

The RFID reader 200 may then attempt a custody transfer of the sensordata on the RFID tag 300, 400 to the sensor controller 500. This beginswith the RFID reader 200 reading the sensor data contained, in oneembodiment, in the memory 312, 414 of the RFID tag 300, 400, as at 810.The sensor data may be read according to the one or more parameters.

The method 800 may also include determining whether the sensor data isread successfully by the RFID reader 200, as at 812. If unsuccessful,the read may be re-attempted a predetermined number of times (e.g.,three times), as at 813. If still unsuccessful after the predeterminednumber of times, the custody transfer attempt may terminate, and themethod 800 may loop back around to step 802.

In one embodiment, if the read is successful before reaching thepredetermined number of times, then the RFID reader 200 may write asecond custody transfer command to the memory 312, 414 of the RFIDsensor tag 300, 400 indicating that the RFID reader 200 has takencustody of the sensor data, as at 814. The method 800 may also includedetermining whether the second custody transfer was successfullywritten, as at 816. If unsuccessful, the write may be re-attempted apredetermined number of times (e.g., three times), as at 817. If stillunsuccessful after the predetermined number of times, the custodytransfer attempt may terminate, the RFID sensor tag 300, 400 may retaincustody of the sensor data packet, and the method 800 may loop backaround to step 802. If it is successful before reaching thepredetermined number of times, then the recovered sensor data may betransmitted from the RFID reader 200 to the sensor controller 500 (orother interested subscribers), as at 820. The interested subscribers mayinclude data consumers such as a display or a database. The method 800then loops back around to step 802.

In this embodiment, the RFID reader 200 may then return to normal RFIDinventory management interrogations until instructed by the sensorcontroller 500 to switch to data transfer mode. In another embodiment,it may attempt several packet data transfers from the RFID tag 300, 400in question before returning to inventory management interrogations.This may be encoded in the custody transfer command from the sensorcontroller 500, or it may be guided by a pre-configured parameter in theRFID interrogator data transfer software implementation. In oneembodiment, it may proceed a fixed number of times before returning toinventory management interrogations, or until the tag 300, 400 hasindicated that has no more data to transfer, whichever comes first. Inanother embodiment, it may proceed until the RFID tag 300, 400 hasindicated it has no more data to transfer. In another embodiment, thesensor controller 500 may attempt data transfers from several RFID tags300, 400 encoded in a single custody transfer message before returningto inventory management interrogations.

FIG. 9 illustrates a flowchart of a method 900 of the RFID tag 300, 400interacting with the method 800, according to an embodiment. The stepsin the method 900 may be performed by the RFID tag(s) 300, 400. Moreparticularly, the method 900 may be stored in the MCU memory 312, 412and acted upon by the MCU 310, 410 of the RFID tag 300, 400. The method900 may operate concurrently with the tag data acquisition methoddescribed in the method 600. In one embodiment, this concurrence isimplemented in a single-threaded MCU 310, 410 using a scheduler. Inanother embodiment, it is implemented in a multi-threaded MCU 310, 410.

During the execution of method 900, the RFID tag 300, 400 may either behibernating or acquiring periodic sensor data, as described in method600. When the RFID reader 200 receives a custody transfer message fromthe RFID sensor controller 500, it may first attempt an RFID memory readof the data packet from the RFID memory 315, 415 of the RFID sensor tag300, 400 as described in the method 800. This process does not generatea notification to the RFID sensor tag 300, 400 upon successfulcompletion, as it is a passive process powered entirely by the RFIDreader 200. Upon successful completion of the RFID memory read, the RFIDreader 200 may then issue an RFID memory write, as described in themethod 800, to pass data to the RFID tag 300, 400 that the reader 200has taken custody of the sensor data packet in the RFID memory 312, 414of the tag 300, 400.

The method 900 may begin with receiving, interpreting, and/or decoding acommand from the RFID reader 200, as at 902. The command may be orinclude an aperiodic custody transfer command.

Once the custody transfer command has been received, the RFID tag 300,400 may then remove a first sensor packet from the packet buffer in thememory 312, 414 of the RFID tag 300, 400, as at 904. The RFID tag 300,400 may then check the sample buffer stored in the memory 312, 412 (anddescribed in method 600) to see if a sensor packet's worth of sampleshave been acquired and stored for later transfer over the RFID network,as at 906. If sufficient samples are not present, the RFID tag 300, 400may write an empty packet indicator to the packet buffer in the memory312, 414, as at 908. The packet buffer may then be marked as empty. Inother words, the packet buffer may be absolved of the responsibility toretain a copy of the current packet data, thereby freeing it to acceptcustody of subsequent packet data. As used herein, “freeing” means toabsolve the packet buffer of the responsibility to retain a copy of thecurrent packet data. If sufficient samples are present, the method 900may include removing an entry from the sample buffer(s) according tosome metric of priority, as described in method 600, encapsulating it asa packet, and writing that packet to the packet buffer of the memory312, 414, as at 910. In one embodiment, a single sample buffer with asingle method of determining priority is present and the method fordetermining packet priority is straightforward.

In another embodiment, multiple sample buffers of varying bufferpriorities, each with its own method for determining priority of thesample entries within the buffer, may be present, and the RFID sensortag 300, 400 may apply higher-level logic to mediate between thesediffering buffer and buffer entry priorities. In one embodiment, only“full” sample buffer entries that contain the maximum number of samplespre-allocated to each packet may be removed from a sample buffer,encapsulated as a packet, and written to the packet buffer in the RFIDmemory 312, 414. In another embodiment, partial entries may be removedfrom a sample buffer, encapsulated as a packet, and written to thepacket buffer in the RFID memory 312, 414 (e.g., in a case where onlyone packet is present in the internal buffer but it is only partiallyfilled at the time a custody transfer command is being processed).Finally, if no sample entries are present and allowed to be removed fromthe sample buffer at the time a custody transfer command is beingprocessed, an “empty” packet may be written to the packet buffer in thememory 312, 414 to indicate that the RFID sensor tag 300, 400 is not inpossession of yet-to-be-transferred sensor data.

What is claimed is:
 1. A system, comprising: a plurality ofradiofrequency identification (RFID) tags including a first RFID tag,the first RFID tag configured to perform RFID tag operations comprising:acquiring one or more samples using a sensor of the first RFID tag;writing the one or more samples in a memory of the first RFID tag; andtransferring custody of the one or more samples to a first RFID readeron request; a plurality of RFID readers including the first RFID reader,the first RFID reader configured to perform RFID reader operationscomprising: reading an identifier from each of the plurality of RFIDtags in view of the first RFID reader during an inventory managementmode; transmitting the identifier from each of the RFID tags in view toa sensor controller during the inventory management mode, whereintransmitting the identifier comprises transmitting a first identifierfor the first RFID tag to the sensor controller; receiving a firstcustody transfer command from the sensor controller based at least inpart on the sensor controller receiving the first identifier; based atleast partially upon receiving the first custody transfer command,reading the one or more samples from the first RFID tag to take custodyof the one or more samples during a custody transfer servicing mode; andduring the custody transfer servicing mode, writing a second custodytransfer command to the memory of the first RFID tag to indicate thatthe first RFID reader has taken custody of the one or more samples;wherein the sensor controller is configured to be in communication withthe plurality of RFID readers, and wherein the sensor controller isconfigured to perform sensor controller operations comprising: receivingthe first identifier from the first RFID reader during the inventorymanagement mode; determining that the first RFID tag is in view of thefirst RFID reader based on receiving the first identifier; andtransmitting the first custody transfer command to the first RFID readerto cause the first RFID reader to request the one or more samples fromthe first RFID tag.
 2. The system of claim 1, wherein the sensorcontroller operations further comprise: receiving the first identifierfrom a second RFID reader of the plurality of RFID readers during theinventory management mode; determining the first RFID reader has anoptimal view of the first RFID tag, wherein the optimal viewdetermination is based on greater signal strength between the first RFIDreader and the first RFID tag than between the second RFID reader andthe first RFID tag; and based on the optimal view determination,selecting the first RFID reader to read the one or more samples from thefirst RFID tag to take custody of the one or more samples during thecustody transfer servicing mode.
 3. The system of claim 2, where sensorcontroller operations further comprise transmitting a command from thesensor controller to the first RFID reader with the optimal view toinstruct the first RFID tag to synchronize a sample clock, to sample thesensor data, and to store the sensor data.
 4. The system of claim 1,wherein the sensor controller operations further comprise: receiving thefirst identifier from a second RFID reader of the plurality of RFIDreaders during the inventory management mode; determining that the firstRFID reader has more available time to divert from the inventorymanagement mode when compared to the second RFID reader; and based onthe available time determination, selecting the first RFID reader toread the one or more samples from the first RFID tag to take custody ofthe one or more samples during the custody transfer servicing mode. 5.The system of claim 1, wherein: the memory has a sample bufferconfigured to store the one or more samples from the sensor; the memoryhas a packet buffer configured to store a first packet including the oneor more samples, the packet buffer being configured to be accessibleover-the-air to transmit the first packet to one or more of theplurality of RFID readers; reading the one or more samples from thefirst RFID tag to take custody of the one or more samples during thecustody transfer servicing mode comprises transmitting the first packetto the first RFID reader; writing the second custody transfer command tothe memory of the first RFID tag to indicate that the first RFID readerhas taken custody of the one or more samples comprises indicating thatthat the first RFID reader has taken custody of the first packet; thepacket buffer is configured to be available for a second packet afterthe first RFID tag receives the second custody transfer command; and thesecond packet from the sample buffer is transferred to the packet bufferin response to the first RFID reader writing the second custody transfercommand to the memory of the first RFID tag.
 6. A method for thecollection of information from a plurality of radio-frequencyidentification (RFID) tags using an RFID system, the method comprising:scanning for the plurality of RFID tags with a plurality of RFIDreaders, wherein the plurality of RFID readers includes a first RFIDreader having a first coverage area and configured to detect one or moreof the plurality of RFID tags when positioned in the first coveragearea; receiving RFID tag identifiers corresponding to the plurality ofRFID tags with the first RFID reader, wherein the RFID tag identifiersinclude a first RFID tag identifier corresponding to a first RFID tag ofthe plurality of RFID tags; transmitting the RFID tag identifiers fromthe first RFID reader to a sensor controller; identifying the first RFIDtag identifier in the RFID tag identifiers using the sensor controller;determining that the first RFID tag is likely to have custody of sensordata; issuing a first custody transfer command from the sensorcontroller to the first RFID reader based at least in part on thedetermination that the first RFID tag is likely to have custody of thesensor data; reading the sensor data from the first RFID tag using thefirst RFID reader in response to the first custody transfer command; andwriting a second custody transfer command from the first RFID reader tothe first RFID tag.
 7. The method of claim 6, wherein the determinationthat the first RFID tag is likely to have custody of the sensor data isat least partially based on information in a database of the sensorcontroller indexed by the first RFID tag identifier.
 8. The method ofclaim 6, wherein the plurality of RFID tags also includes a second RFIDreader having a second coverage area and configured to detect one ormore of the plurality of RFID tags when positioned in the secondcoverage area, wherein the method further comprises: receiving the RFIDtag identifiers corresponding to the plurality of RFID tags with thesecond RFID reader, wherein the RFID tag identifiers include the firstRFID tag identifier corresponding to the first RFID tag of the pluralityof RFID tags; transmitting the RFID tag identifiers from the second RFIDreader to the sensor controller; identifying the first RFID tagidentifier in the RFID tag identifiers from the first and second RFIDreaders using the sensor controller; and determining that the first RFIDreader has a more optimal view of the first RFID tag than the secondRFID reader.
 9. The method of claim 8, wherein the first custodytransfer command is issued to the first RFID reader and not the secondRFID reader based upon the determination that the first RFID reader hasthe more optimal view of the first RFID tag than the second RFID reader.10. The method of claim 8, further comprising transmitting a commandfrom the sensor controller to the first RFID reader with the moreoptimal view to instruct the first RFID tag to synchronize a sampleclock, to sample the sensor data, and to store the sensor data.
 11. Asystem, comprising: a radiofrequency identification (RFID) tagconfigured to perform RFID tag operations, the RFID tag operationscomprising: transferring custody of data in a packet buffer of a memoryof the RFID tag to an RFID reader, thereby freeing the packet buffer toreceive new data; acquiring one or more samples using a sensor of theRFID tag; storing the one or more samples in a sample buffer of thememory of the RFID tag; removing a first entry in the sample buffer whenthe first entry contains greater than or equal to a threshold number ofthe one or more samples; encapsulating the first entry as a packet,wherein the packet comprises the one or more samples and packetmetadata; placing the packet in the packet buffer in response to acustody transfer request from the RFID reader; adding an empty entry tothe packet buffer when the sample buffer contains less than thethreshold number of the one or more samples; and allowing acquisition ofnew samples into a second entry in the sample buffer concurrently withprocessing a new custody transfer request from the RFID reader.
 12. Thesystem of claim 11, wherein transferring custody comprises: receiving acustody transfer command from the RFID reader; authenticating thecustody transfer command with respect to the packet in custody; andmarking the packet buffer as empty.
 13. The system of claim 11, whereinthe RFID tag operations further comprise: receiving a wake-up signalfrom the RFID reader; extracting a command from the wake-up signal;determining that the command comprises a periodic action; synchronizinga clock in the RFID tag with a clock in the RFID reader in response todetermining that the command comprises the periodic action; and startinga timer in the RFID tag using the synchronized clock, wherein the one ormore samples are acquired before the timer expires, and wherein the oneor more samples are written into the current entry in the sample bufferafter the timer expires.
 14. The system of claim 13, wherein the RFIDtag operations further comprise determining whether the packet bufferhas the packet in custody in response to the sample buffer containinggreater than or equal to the threshold number of the one or moresamples.
 15. The system of claim 14, wherein, when the packet bufferdoes not have the packet in custody, the RFID tag operations furthercomprise: removing the buffer entry from the sample buffer;encapsulating the buffer entry as the packet; and writing the packet tothe packet buffer.
 16. The system of claim 11, further comprising theRFID reader, which is configured to perform RFID reader operations, theRFID reader operations comprising: receiving a first custody transfercommand for the RFID tag from a sensor controller; extracting one ormore parameters from the first custody transfer command; reading the oneor more samples from the RFID tag based at least partially upon the oneor more parameters; and writing a second custody transfer command to acommand buffer of the memory of the RFID tag to indicate that the RFIDreader has taken custody of the one or more samples.
 17. The system ofclaim 16, wherein the one or more samples comprise one or more physicalenvironmental measurements.
 18. The system of claim 16, wherein the RFIDreader operations further comprise: repeating reading the one or moresamples and writing the second custody transfer command until either anempty buffer entry is read or the RFID reader is compelled to return toan inventory management mode after expiration of a timer governing timespent outside of the inventory management mode.
 19. The system of claim11, further comprising a sensor controller configured to perform sensorcontroller operations, the sensor controller operations comprising:receiving one or more parameters related to an interrogation of the RFIDtag by the RFID reader; identifying an identifier of the RFID tag in theone or more parameters; determining that a likelihood that the RFID taghas custody of the one or more samples is greater than a threshold basedat least partially upon the identifier; determining that the RFID readeris in range to communicate with the RFID tag; transmitting a custodytransfer command to the RFID reader to cause the RFID reader to requestthe one or more samples from the RFID tag; and receiving the one or moresamples from the RFID reader.
 20. The system of claim 19, wherein thesensor controller operations further comprise causing the one or moresamples to be displayed on a monitor, saved into a database, or both.